Fish Keeping

A fish (: fish or fishes) is an aquatic, anamniotic, gill-bearing vertebrate animal with swimming fins and a hard skull, but lacking limbs with digits. Fish can be grouped into the more basal jawless fish and the more common jawed fish, the latter including all living cartilaginous and bony fish, as well as the extinct placoderms and acanthodians. In a break to the long tradition of grouping all fish into a single class (Pisces), modern phylogenetics views fish as a paraphyletic group.

Most fish are cold-blooded, their body temperature varying with the surrounding water, though some large active swimmers like white shark and tuna can hold a higher core temperature. Many fish can communicate acoustically with each other, such as during courtship displays. The study of fish is known as ichthyology.

The earliest fish appeared during the Cambrian as small filter feeders; they continued to evolve through the Paleozoic, diversifying into many forms. The earliest fish with dedicated respiratory gills and paired fins, the ostracoderms, had heavy bony plates that served as protective exoskeletons against invertebrate predators. The first fish with jaws, the placoderms, appeared in the Silurian and greatly diversified during the Devonian, the "Age of Fishes".

Bony fish, distinguished by the presence of swim bladders and later ossified endoskeletons, emerged as the dominant group of fish after the end-Devonian extinction wiped out the apex predators, the placoderms. Bony fish are further divided into the lobe-finned and ray-finned fish. About 96% of all living fish species today are teleosts, a crown group of ray-finned fish that can protrude their jaws. The tetrapods, a mostly terrestrial clade of vertebrates that have dominated the top trophic levels in both aquatic and terrestrial ecosystems since the Late Paleozoic, evolved from lobe-finned fish during the Carboniferous, developing air-breathing lungs homologous to swim bladders. Despite the cladistic lineage, tetrapods are usually not considered to be fish.

Fish have been an important natural resource for humans since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or in breeding cages in the ocean. Fish are caught for recreation, or raised by fishkeepers as ornaments for private and public exhibition in aquaria and garden ponds. Fish have had a role in human culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Etymology

The word ''fish'' is inherited from Proto-Germanic, and is related to German , the Latin and Old Irish , though the exact root is unknown; some authorities reconstruct a Proto-Indo-European root , attested only in Italic, Celtic, and Germanic.

Evolution

Fossil history

About 530 million years ago during the Cambrian explosion, fishlike animals with a notochord and eyes at the front of the body, such as '' Haikouichthys'', appear in the fossil record. During the late Cambrian, other jawless forms such as conodonts appear.

Jawed vertebrates appear in the Silurian, with giant armoured placoderms such as '' Dunkleosteus''. Jawed fish, too, appeared during the Silurian: the cartilaginous Chondrichthyes and the bony Osteichthyes.

During the Devonian, fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes".

Phylogeny

Fishes are a paraphyletic group, since any clade containing all fish, such as the Gnathostomata or (for bony fish) Osteichthyes, also contains the clade of tetrapods (four-limbed vertebrates, mostly terrestrial), which are usually not considered fish. Some tetrapods, such as cetaceans and ichthyosaurs, have secondarily acquired a fish-like body shape through convergent evolution. On the other hand, ''Fishes of the World'' comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods". The biodiversity of extant fish is unevenly distributed among the various groups; teleosts, bony fishes able to protrude their jaws, make up 96% of fish species. The cladogram shows the evolutionary relationships of all groups of living fishes (with their respective diversity) and the tetrapods. Table 1a
Number of species evaluated in relation to the overall number of described species, and numbers of threatened species by major groups of organisms
/ref> Extinct groups are marked with a dagger (†); groups of uncertain placement are labelled with a question mark (?) and dashed lines (- - - - -).

Taxonomy

Fishes (without tetrapods) are a paraphyletic group and for this reason, the class ''Pisces'' seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three extant classes ("Agnatha", Chondrichthyes, and "Osteichthyes"), and with extinct forms sometimes classified within those groups, sometimes as their own classes.

Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within the nine largest families; from largest to smallest, these are Cyprinidae, Gobiidae, Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families are monotypic, containing only one species.

Diversity

Fish range in size from the huge whale shark to some tiny teleosts only long, such as the cyprinid '' Paedocypris progenetica'' and the stout infantfish.

File:Rhincodon typus fgbnms (cropped).jpg, Largest: whale shark
File:Paedocypris progenetica 001.jpg, Smallest: e.g. '' Paedocypris progenetica''

Swimming performance varies from fish such as tuna, salmon, and jacks that can cover 10–20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second.

File:Salmo salar.jpg, Fastest: e.g. salmon, 10–20 body lengths/second
File:Anguilla japonica 1856.jpg, Slowest: e.g. eel, 0.5 body lengths/second

A typical fish is cold-blooded, has a streamlined body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and a tail fin, jaws, skin covered with scales, and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes.

File:Humpback anglerfish.png, Ambush predator:
anglerfish
File:Atl mackerel photo3 exp.jpg, Streamlined, somewhat warm-blooded:
mackerel
File:Hippocampus hippocampus (cropped).jpg, Tail not used for swimming:
seahorse
File:Phycodurus eques P2023146 (cropped).JPG, Camouflaged:
leafy seadragon
File:Eastern Cleaner Clingfish (cropped).jpg, No scales:
clingfish
File:Cyphotilapia frontosa mouthbrooding.jpg, Mouthbrooder: front cichlid with young in mouth

Ecology

Habitats

Fish species are roughly divided equally between freshwater and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species. Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo, and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth.

Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., cusk-eels and snailfish), although none have been found in the deepest 25% of the ocean. The deepest living fish in the ocean so far found is a cusk-eel, '' Abyssobrotula galatheae'', recorded at the bottom of the Puerto Rico Trench at .

In terms of temperature, Jonah's icefish live in cold waters of the Southern Ocean, including under the Filchner–Ronne Ice Shelf at a latitude of 79°S, while desert pupfish live in desert springs, streams, and marshes, sometimes highly saline, with water temperatures as high as 36 C.

A few fish live mostly on land or lay their eggs on land near water. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows. A single undescribed species of '' Phreatobius'' has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged leaf litter. Cavefish of multiple families live in underground lakes, underground rivers or aquifers.

Parasites and predators

Like other animals, fish suffer from parasitism. Some species use cleaner fish to remove external parasites. The best known of these are the bluestreak cleaner wrasses of coral reefs in the Indian and Pacific oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract the attention of the cleaners. Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, '' Etroplus maculatus'', the cleaner, and the much larger '' E. suratensis''.

Fish occupy many trophic levels in freshwater and marine food webs. Fish at the higher levels are predatory, and a substantial part of their prey consists of other fish. In addition, mammals such as dolphins and seals feed on fish, alongside birds such as gannets and cormorants.

File:Initial phase parrotfish feeding at Shaab Marsa Alam, Red Sea, Egypt -SCUBA (6336981391).jpg, A parrotfish feeding on algae on a coral reef
File:Arothron hispidus is being cleaned by Hawaiian cleaner wrasses, Labroides phthirophagus 1.jpg, A cleaner fish removing parasites from its client, a pufferfish
File:Barracuda with prey.jpg, A barracuda preying on a smaller fish
File:1031 california sealion wright odfw (35281910502).jpg, Sea lion, a predatory mammal, eating a large salmonid
File:Cormorant with fish (cropped).jpg, Cormorant with fish prey

Anatomy and physiology

Locomotion

The body of a typical fish is adapted for efficient swimming by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as control surfaces like an aircraft's flaps, enabling the fish to steer in any direction.

File:Lampanyctodes hectoris (Hector's lanternfish).svg, Anatomy of a typical fish ( lanternfish shown):
1) gill cover 2) lateral line 3) dorsal fin 4) fat fin
5) caudal peduncle 6) caudal fin 7) anal fin 8)  photophores 9) pelvic fins 10) pectoral fins

Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that allows them to adjust their buoyancy by increasing or decreasing the amount of gas it contains.

The scales of fish provide protection from predators at the cost of adding stiffness and weight. Fish scales are often highly reflective; this silvering provides camouflage in the open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility.

File:Swim bladder.jpg, Gas-filled swim bladder of a rudd helps maintain neutral buoyancy.
File:Fish scales.jpg, Silvered scales of a rohu provide protection and camouflage.

Circulation

Fish have a closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through the gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart.

Respiration

Gills

Fish exchange gases using gills on either side of the pharynx. Gills consist of comblike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in the gills flows in the opposite direction to the water, resulting in efficient countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath a protective bony cover or operculum. They are able to oxygenate their gills using muscles in the head.

Air breathing

Some 400 species of fish in 50 families can breathe air, enabling them to live in oxygen-poor water or to emerge on to land. The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to the heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of rock pools and the intertidal zone, are facultative air breathers, able to breathe air when out of water, as may occur daily at low tide, and to use their gills when in water. Some coastal fish like rockskippers and mudskippers choose to leave the water to feed in habitats temporarily exposed to the air. Some catfish absorb air through their digestive tracts.

Digestion

The digestive system consists of a tube, the gut, leading from the mouth to the anus. The mouth of most fishes contains teeth to grip prey, bite off or scrape plant material, or crush the food. An esophagus carries food to the stomach where it may be stored and partially digested. A sphincter, the pylorus, releases food to the intestine at intervals. Many fish have finger-shaped pouches, pyloric caeca, around the pylorus, of doubtful function. The pancreas secretes enzymes into the intestine to digest the food; other enzymes are secreted directly by the intestine itself. The liver produces bile which helps to break up fat into an emulsion which can be absorbed in the intestine.

Excretion

Most fish release their nitrogenous wastes as ammonia. This may be excreted through the gills or filtered by the kidneys. Salt is excreted by the rectal gland. Saltwater fish tend to lose water by osmosis; their kidneys return water to the body, and produce a concentrated urine. The reverse happens in freshwater fish: they tend to gain water osmotically, and produce a dilute urine. Some fish have kidneys able to operate in both freshwater and saltwater.

Brain

Fish have small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, notably mormyrids and sharks, which have brains about as large for their body weight as birds and marsupials. At the front of the brain are the olfactory lobes, a pair of structures that receive and process signals from the nostrils via the two olfactory nerves. Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the telencephalon, which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to the midbrain is the diencephalon; it works with hormones and homeostasis. The pineal body is just above the diencephalon; it detects light, maintains circadian rhythms, and controls color changes. The midbrain contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids. The hindbrain controls swimming and balance.The single-lobed cerebellum is the biggest part of the brain; it is small in hagfish and lampreys, but very large in mormyrids, processing their electrical sense. The brain stem or myelencephalon controls some muscles and body organs, and governs respiration and osmoregulation.

Sensory systems

The lateral line system is a network of sensors in the skin which detects gentle currents and vibrations, and senses the motion of nearby fish, whether predators or prey. This can be considered both a sense of touch and of hearing. Blind cave fish navigate almost entirely through the sensations from their lateral line system. Some fish, such as catfish and sharks, have the ampullae of Lorenzini, electroreceptors that detect weak electric currents on the order of millivolt.

Vision is an important sensory system in fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rods and cones (for scotopic and photopic vision); many species have colour vision, often with three types of cone. Teleosts can see polarized light; some such as cyprinids have a fourth type of cone that detects ultraviolet. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. See als
Lamb et al.'s "The origin of the Vertebrate Eye", 2008.
/ref>

Hearing too is an important sensory system in fish. Fish sense sound using their lateral lines and otoliths in their ears, inside their heads. Some can detect sound through the swim bladder.

Some fish, including salmon, are capable of magnetoreception; when the axis of a magnetic field is changed around a circular tank of young fish, they reorient themselves in line with the field. The mechanism of fish magnetoreception remains unknown; experiments in birds imply a quantum radical pair mechanism.

Cognition

The cognitive capacities of fish include self-awareness, as seen in mirror tests. Manta rays and wrasses placed in front of a mirror repeatedly check whether their reflection's behavior mimics their body movement. '' Choerodon'' wrasse, archerfish, and Atlantic cod